Photo: Rensselaer Polytechnic Institute |
Researchers
at Rensselaer Polytechnic Institute have developed a new method for
manufacturing green-colored LEDs with greatly enhanced light output.
The
research team, led by Christian Wetzel, professor of physics and the Wellfleet
Constellation Professor of Future Chips at Rensselaer,
etched a nanoscale pattern at the interface between the LED’s sapphire base and
the layer of gallium nitride (GaN) that gives the LED its green color. Overall,
the new technique results in green LEDs with significant enhancements in light
extraction, internal efficiency, and light output.
The
discovery brings Wetzel one step closer to his goal of developing a
high-performance, low-cost green LED.
“Green
LEDs are proving much more challenging to create than academia and industry
ever imagined,” Wetzel said. “Every computer monitor and television produces
its picture by using red, blue, and green. We already have powerful,
inexpensive red and blue LEDs. Once we develop a similar green LED, it should
lead to a new generation of high-performance, energy-efficient display and
illumination devices. This new research finding is an important step in the
right direction.”
Sapphire
is among the least expensive and widely used substrate materials for
manufacturing LEDs, so Wetzel’s discovery could hold important implications for
the rapidly growing, fast-changing LED industry. He said this new method should
also be able to increase the light output of red and blue LEDs.
Results
of the study, titled “Defect-reduced green GaInN/GaN light-emitting diode on
nanopatterned sapphire,” were published in Applied
Physics Letters, and are featured in the Virtual Journal of Nanoscale Science &
Technology. The paper may be viewed online at: http://dx.doi.org/10.1063/1.3579255.
The
research program is supported by the U.S. Department of Energy National Energy
Technology Laboratory (NETL) Solid-State Lighting Contract of Directed
Research, and the National Science Foundation (NSF) Smart Lighting Engineering
Research Center (ERC), which is led by Rensselaer.
LED
lighting only requires a fraction of the energy required by conventional light
bulbs, and LEDs contain none of the toxic heavy metals used in the newer
compact fluorescent light bulbs. In general, LEDs are very durable and
long-lived.
First
discovered in the 1920s, LEDs are semiconductors that convert electricity into
light. When switched on, swarms of electrons pass through the semiconductor
material and fall from an area with surplus electrons into an area with a
shortage of electrons. As they fall, the electrons jump to a lower orbital and
release small amounts of energy. This energy is realized as photons—the most
basic unit of light. Unlike conventional light bulbs, LEDs produce almost no
heat.
The
color of light produced by LEDs depends on the type of semiconductor material
it contains. The first LEDs were red, and not long thereafter researchers
tweaked their formula and developed some that produced orange light. Years
later came blue LEDs, which are frequently used today as blue light sources in
mobile phones, CD players, laptop computers, and other electronic devices.
The
holy grail of solid-state lighting, however, is a true white LED, Wetzel said.
The white LEDs commonly used in novelty lighting applications, such as key
chains, auto headlights, and grocery freezers, are actually blue LEDs coated
with yellow phosphorus—which adds a step to the manufacturing process and also
results in a faux-white illumination with a noticeable bluish tint.
The
key to true white LEDs, Wetzel said, is all about green. High-performance red
LEDs and blue LEDs exist. Pairing them with a comparable green LED should allow
devices to produce every color visible to the human eye—including true white,
Wetzel said. Today’s computer monitor and television produces its picture by
using red, blue, and green. This means developing a high-performance green LED
could therefore likely lead to a new generation of high-performance,
energy-efficient display devices.
The problem, however, is that green LEDs
are much more difficult to create than anyone anticipated. Wetzel and his
research team and investigating how to “close the green gap,” and develop green
LEDs that are as powerful as their red or blue counterparts.